1. Drawbacks of OFDM Modulation
A major drawback of the OFDM technique is inherent in the strong fluctuations in amplitude of the envelope of the modulated signal and therefore in high variations in instantaneous power.
The peak-to-average power ratio (PAPR) of the signals sent is thus generally very high and increases with the number of subcarriers N.
Power amplifiers have non-linear characteristics which, coupled with the amplification of the so-called high PAPR signals, lead to distortions: spectral regrowth of the secondary-lobes level, generation of harmonics, creation of interference between non-linear symbols, creation of interference between carriers. Thus these distortions give rise in particular to transmission errors and degradation in the bit error rate (BER).
2. Prior Art for Reduction in the PAPR
In the literature, numerous techniques have already been proposed for overcoming this problem.
A usual solution consists of ensuring that the operating range of the amplifier remains limited to a linear amplification region, which unfortunately limits the efficiency of the amplifier (a few % instead of, conventionally, 50%) and therefore a significant increase in the power consumption of the emitter. This is a very high constraint in the use of OFDM, in particular in mobile terminals, since the consumption of the power amplifier may represent more than 50% of the total consumption of a terminal.
A second approach is the “clipping” technique, which consists of clipping the amplitude of the signal when it exceeds a predefined threshold. However, this clipping is by nature non-linear and introduces a distortion in the signal sent, resulting not only in a degraded BER but also a regrowth of secondary lobes of the PSD (Power Spectral Density).
A third technique, referred to as “selected mapping”, consists of applying a phase rotation to each symbol in the sequence to be transmitted. Several phase rotation patterns may be defined. For each pattern applied to the sequence to be transmitted, the operations are performed for obtaining a corresponding OFDM signal, and the one having the lowest PAPR is transmitted. Once again this technique does not give any distortion, but it requires communicating to the receiver the rotation sequence used on transmission with very high reliability, which leads to a reduction in the spectral efficiency and a significant increase in the complexity of the system for routing the number of the pattern used via a dedicated channel. In addition, if this transmission is erroneous, the entire OFDM frame will be lost. It also increases the complexity on transmission, since several processing operations must be performed in parallel, in order then to choose the most effective. The other processing operations have been performed unnecessarily, and are not used.
A fourth method, usually referred to as “TR technique” (Tone Reservation), proposes to reserve certain subcarriers of the OFDM multiplex, which do not transport information but symbols optimised on transmission in order to reduce the PAPR. These symbols can be optimised by using for example a convex optimisation algorithm of the SOCP (Second Order Cone Programming) type. Just like the previous method, this solution does not cause any distortion to the transmitted signal, but a major drawback of this method lies in the fact that a certain number of carriers must be reserved to be able to reduce the PAPR significantly. These carriers are not used for transmitting useful information data, which leads to a reduction in the spectral efficiency.
This reduction in the spectral efficiency may be solved by the use of so-called “pilot” carriers, initially dedicated to channel estimation, for reducing the PAPR, with on reception a so-called “blind” detection, as proposed in the document “Peak Power Reduction for OFDM Systems With Orthogonal Pilot Sequences” (M. J. Fernandes-Getino Garcia, O. Edfors, J. M. Paez-Borrallo, IEEE Transactions on Wireless Communications, Volume 5, January 2006, pages 47-51).
This document describes such a technique applied to orthogonal pilot sequences, in particular Walsh-Hadamard sequences, which consequently imposes the choice of pilot carriers in a set of two distinct values (−1 and 1) predefined on transmission, so that the sequences are orthogonal to each other.
However, the efficacy of such a technique is limited. Indeed, this technique imposes on transmission a limited choice of pilot carrier modifications in a set of predefined values, thus limiting the performance of this technique in terms of PAPR reduction. Indeed, the pilot carriers modified by this technique are those that best reduce the PAPR, among predefined values, rather than those that reduce the PAPR optimally.